The invention relates to a device for the supply and viscosity regulation of fuel, intended for use particularly in thermal engines.
The thermal engines are often designed and fed so as to run on light fuel during starting and at low load, and on heavy fuel oil at nominal speed.
Usually, for light fuel feed, a simple piping directly connects the corresponding tank to the engine injection device.
On the contrary, as regards heavy fuel oil, before it reaches the engine injection device, its viscosity must be corrected. Indeed, the heavy fuel oil can reach a very high viscosity and if this fuel was brought unmodified to the engine injection device, the pressures which should be applied for its injection would destroy the injection device.
It is known to correct the viscosity of the heavy fuel oil by more or less intense heating, under the control of a viscosimetric device.
Because of its heterogeneousness, however, the heavy fuel oil reaching the heating device has a viscosity which very often and suddenly varies considerably. Thus, the correction determined by the viscosimetric device very often risks to be irrelevant to the correction which has become necessary, thus resulting in very uncertain running conditions.
Further, because the above-noted adjustment of viscosity is effected exclusively through temperature, changes the fuel reaches the engine with widely variable temperatures. Moreover, during the change over from light fuel feeding to heavy fuel feeding, as well as during the change back to light fuel the engine must run in an intermediate stage during which it is generally not possible to monitor the temperature and the viscosity of the blend. It was found that during these intermediate stages malfunction may occur: the rapid temperature variation of the fuel called `thermal shock` causes blocking of the injection pumps and the abnormal increase of viscosity during short periods damages the injection equipment and the camshafts. According to another known method, the fuel viscosity is corrected, besides heating, by blending both fuels in proportions which can be modified according to the data which a viscosimetric device transmits to an electric or electronic arrangement determining the new proportion and/or setting the same by adjusting the flow in at least one of the two pipes which carry the heavy fuel oil and the light fuel respectively. Such a method is disclosed in U.S. Pat. No. 3,415,264.
For such a flow regulation in known constructions, each pipe must pass through a separate pump, one of which, at least, either has a variable output or is rotated by a variable-speed engine.
Apart from the fact that such pumps are very expensive and prevent a gravity flow of the fuel in case of shut down of their motor, when both pumps are acted upon at the same time, the new proportion is difficult to determine and, when only one pump is acted upon, the possibilities of blend modification are limited.